The present invention relates to a composite material for the protection of sensitive devices to the permeation of H2O from the external environment, the composite material being formed of nanozeolites dispersed in a polymeric matrix.
It is known that the presence of H2O, even in the form of traces, is harmful for the correct operation of several devices, among which there are micro-electromechanical devices known in the field as MEMS (Micro-Electro-Mechanical Systems), OLED type organic displays (Organic Light Emitting Diode) and photovoltaic cells such as OSC (Organic Solar Cells) or DSSC (Dye-Sensitized Solar Cells), to mention some of the most interesting ones. In the following, reference will be made to these devices and more generally to any sealed device wherein the presence of H2O, even in small amounts (lower than 5000 ppm), results to be harmful, with the wording “sensitive device”.
The presence of H2O in sensitive devices may cause the progressive deterioration of their performances. More information as to the effects of this contaminant may be found in the scientific paper “Correlation between dark spot growth and pinhole size in organic light-emitting diodes”, by Shuang Fang Lim, et al., published on Applied Physics Letters, Vol. 78 No. 15, on 9 Apr. 2001 concerning OLED displays, and in the fifth chapter of the book “Organic Photovoltaics-Concepts and Realization” by Brabec et al., published in 2003 by Springer-Verlag, concerning OSC type photovoltaic cells.
The use of sorbents for the removal of gaseous impurities from the housings of devices sensitive to their presence is known in the art. For example, the international patent publication WO 2004/072604 in the applicant's name discloses the use of active components dispersed in suitable porous matrices; on the other hand international patent publications WO 2007/013118 and WO 2007/013119, both in the applicant's name, disclose nanostructured systems in which the active component is restricted in a porous means that are dispersed in a polymeric matrix, while the use of functionalized nuclei dispersed in a permeable polymeric matrix is disclosed in the international publication WO 2007/074494, also in the applicant's name.
However, the use of the above-described solutions may be not sufficient to extend the life of the sensitive device up to a length of time compatible with its application field, especially when the permeation flux of H2O is at a higher speed than the gettering flux specified by the sorbing properties of the composite getter. The permeation speed from the external environment in fact depends on how the device is sealed during the manufacturing step, i.e. on the chemical-physical characteristics of the material used for such a sealing.
During the whole life of the device, the sealing material must be able to act as a barrier in order to protect the packaged device from possible contaminants coming from the outside environment. If the permeation of these contaminants through the sealant is such to lead to the saturation of the getter material inserted in the device after a time that is too short or, anyway, not compatible with the intended application of the device, its protection from possible deterioration processes is not ensured: the barrier shall possess suitable characteristics in term of water permeation in order to guarantee an acceptable lifetime and the presence of the getter material in the sensitive device may have a synergistic effect with the barrier, avoiding its premature saturation. Alternatively, the protection can not be ensured when the effective permeation speed into the device is higher than the sorbing speed of the getter material. Even when the getter material is not inactive due to the saturation if the contaminant is captured with a sorption speed lower than the permeation speed, the damage of the sensitive device element can not be avoided.
In the manufacturing processes of sensitive devices, the use of different polymeric materials to ensure sealing and the protection of the devices from the outer environment, due to their low permeability to the different species of contaminants, is known in the art. For example, the international publication WO 2003/011939 in the name of 3M Innovative Properties Co. discloses that epoxy resins and organic compounds comprising hydroxylic groups can be used as a base for chemical compositions that after a consolidation treatment allow to obtain surface adhesion and low coefficients of moisture transmission rate (commonly referred to as WVTR-Water Vapor Transmission Rate).
Among the suitable additives in these compositions, the use of inert inorganic materials is foreseen in order to reduce the permeation of H2O in the enclosed environment of the device. However, the use of inert materials limits the barrier properties essentially to those of the final polymeric matrix, and the publication does not tackle the technical problem of achieving a homogeneous dispersion of these inorganic materials associated with the preparation of the composition
Other drawbacks in the case of barriers consisting of a polymeric matrix containing sorbing species featuring a micrometric particle size are given by the difficulties of obtaining sufficiently homogeneous deposits during the manufacturing step of the device especially if thicknesses have to be of about 15 μm or lower. This is coupled with additional difficulties of ensuring a sufficient adhesion to the involved surfaces of the device or of the encapsulant structure after the consolidation process. Moreover, when sealing materials contain sorbing species featuring a nanometric particle size (nanozeolites in particular) the limits are more commonly represented by the difficulty of ensuring a homogeneous dispersion of these sorbing species inside the polymeric matrix due to their tendency to agglomerate caused by their large surface area. The difficulty to control the agglomeration phenomena could cause difficulties to obtain reproducible final properties of the consolidated material and therefore to properly use it in industrial manufacturing processes.
A possible solution for the these composite materials is disclosed in the international publication WO 2008/057045 in the name of Agency For Science Technology And Research, which describes the use of reactive nanoparticles able to sorb water and oxygen molecules inside an epoxy-based composition. However, this document teaches the use of this composition within a multilayer structure able to obtain low permeation values from the outside of the sensitive device. In fact, it is well known in the encapsulation technical field of sensitive devices that better barrier properties are obtained when using one or more sorbing layers (based on a polymeric-getter composite materials or only on getter materials) externally coupled with a so-called (strictly speaking) “sealing layer”, i.e. based on a low permeable material but not containing a reactive species. Anyway, these multilayer structure usually need a manufacturing process that is more complex than using a single-layer barrier.
The article “Characterization and preparation of epoxy resin/zeolite nanocomposite” by Chen et al, edited on Diqiu Kexue vol. 27, pp. 403-407 in 2002 teaches on the use of surfactants or templating agents in order to overcome the difficulties to obtain an homogeneous dispersion of nanoparticles in a polymeric matrix. Similarly, other scientific articles, as for example “Preparation and Characterization of epoxy composites filled with functionalized nano-sized MCM-41 particles” by Wang et al., edited on Journal of Material Sciences, vol. 43, pp. 3683-3688, on 4 Apr. 2008, have studied the effect of inorganic functionalizations or templating agents to improve the interaction between the inorganic particles and the polymeric matrix. All these publications describe different approaches to improve the control on the interaction of the inorganic surface with the organic phase.
As described in the International Patent Application published as WO 2008/000457, the above mentioned nanozeolites can be coated on their surface, where the chemical surface modification is characterized by hydrophobic organic groups bonded by covalent bonds to the zeolite surface. This publication describes some possible applications (related to the uniformity of the nanozeolites in the organic material) for the final composite material that contains them. In particular such reference addresses the problem of the uniformity of the distribution but is completely silent on the effect of the functionalization on the absorption properties of the zeolites and on the quantitative evaluation of its effect on the barrier properties of the sealing material.